Parasites are ubiquitous in nature, with the potential to affect variation within and among species in ecological networks. My work aims to understand how variation within the food web affects parasitism and, conversely, how parasites can affect species interactions within food webs.
How Parasites Affect Species Interactions
All organisms are vulnerable to parasites, and infection with parasites can have multifarious effects on host organisms, ranging from mild to severe. Classically, these effects of parasites have been considered in a vacuum, ignoring the complexity of natural systems and the numerous interactions that host organisms engage in, such as those with predators and competitors. I aim to investigate the potential of parasites to affect the outcome of these species interactions within a meta-analytical framework, with the goal of explaining more of variation in species interactions in ecological networks.
Cascading Effects of Predators and Immune Function
Communities vary in predator composition, and these predators will differentially affect the life-history strategies of the prey species that live in those environments. In our system, Enallagma spp. damselflies inhabit lakes with either fish or large dragonfly larvae as the top predator, and as such these damselflies will experience different selection pressures. Specifically, damselflies in fish lakes respond to the threat of fish predation by reducing their activity rates, and this reduction is known to affect intraspecific competition. Changes in competition affect resource acquisition, which should in turn affect damselfly immune function. My work aims to determine how the cascading effects of predators affects variation in host immune function, which may then extend to effects on parasite prevalence and intensity.
Temporal Variation in Parasite Prevalence
Species interactions are not static and will vary over time. Parasitism is no exception, and as a result parasite prevalence (the proportion of the host population infected with parasites) is not a constant in natural systems. Besides the immediate effects this variation has on the host organism through reduced risk of infection, these differences have the potential to scale up to community-levels effects. Over the course of three summer field seasons, members of the Siepielski lab and I visited various lakes in northwest and west-central Arkansas and northeast Oklahoma, collecting adult odonates from both fish- and fishless lakes. These different lakes have a variety of unique species assemblages, predator types, and habitat, giving a more robust and complete representation of the ecology of this system. Collected samples are processed back at the lab to measure both the prevalence and intensity of infection at the various sites. I am investigating how the prevalence and intensity of parasitism changes over time, and what factors may explain these changes.
Density and Host Species Specificity
Density has been shown to affect parasitism and infection rates, with more numerous host populations supporting greater parasite abundances, and organisms that live in more aggregated and dense populations are more susceptible to parasites. Many parasite species infect specific host species, though other species are more generalist in their host infection dynamics. In our system, Enallagma signatum is both the most abundant and the most commonly parasitized species. Additionally, E. signatum has the weakest immune system and is therefore least able to resist any parasite attack, which may make them prime targets for their water mite parasites. To understand if it is E. signatum density or species specificity that is driving parasite prevalence, I performed an experiment using both E. signatum and E. basidens, a common species that co-occurs with E. signatum, altering the presence and densities of the hosts, to disentangle these effects.